In a typical wireless communication system, a wireless communication device associated with the system and wishing to communicate through the system needs to transmit a random access (RA) message (or similar) to the system, thereby requesting radio resources for communication.
In some systems, the random access message itself may be able to carry some data (apart from the random access request) from the wireless communication device and may, thus, be used to convey smaller amount of data without a connection having to be set up.
The random access message should typically be transmitted over a random access channel (RACH) defined in association with the wireless communication system. To be able to correctly use the random access channel, a wireless communication device is required to have a certain time and/or frequency synchronization accuracy vis-à-vis the system (e.g. a network node of the system).
In a typical example, synchronization in time needs to be achieved at symbol level (order of microseconds) and synchronization in frequency needs to be achieved at a level of 0.1 ppm (200 Hz for the case of a 2 GHz carrier). Synchronization is typically achieved by having the wireless communication device listen to the downlink from a network node at regular time intervals (order of seconds) even when the wireless communication is not actively connected to the system.
In emerging wireless communication concepts (e.g. the 5th generation cellular communication standard advocated by the Third Generation Partnership Project—3GPP), one application considers support of reliable ultra-low delay machine-type communication (MTC), a.k.a. Critical-MTC. The Critical-MTC concept may need to address design trade-offs regarding e.g., end-to-end latency, transmission reliability, system capacity and deployment. According to some scenarios, the Critical-MTC system should provide for radio resource management that allows coexistence between different classes of applications, for example, applications with sporadic data requiring ultra-low latency (e.g., alert messages) along with applications with real-time periodic data requiring normal latency and/or applications with data under best-effort requirements.
Alert messages (e.g. alarms) represent one potentially important type of messages that may benefit from the Critical-MTC application. Alarms are typically rare events. Hence, the wireless communication device (e.g. a sensor) transmitting the alarm may be of a type associated with low power and/or low cost. Typically, such a type of wireless communication device may not be actively connected or synchronized to the network node continuously. For example, it may be battery driven with a requirement that the battery should last for a long period of time (order of years) and such a requirement may be impossible to fulfill (due to the associated power consumption) if repeatedly listening to the downlink for synchronization. Thus, when an alarm occurs, the wireless communication device first needs to acquire the appropriate synchronization before it may transmit the alarm using the random access channel, which may substantially delay the transmission more than required by the ultra-low delay criterion of Critical-MTC.
Therefore, there is a need for random access approaches with low latency which can be used by a wireless communication device with low power requirements.